docs: def all_label_values(self, label_list_ids=None):
          """
          Return a set of all label-values occurring in this utterance.
  
          Args:
              label_list_ids (list): If not None, only label-values from
                                     label-lists with an id contained in this list
                                     are considered.
  
          Returns:
               :class:`set`: A set of distinct label-values.
          """
          values = set()
  
          for label_list in self.label_lists.values():
              if label_list_ids is None or label_list.idx in label_list_ids:
                  values = values.union(label_list.label_values())
  
          return values